The present invention relates to a limiting current-type oxygen sensor utilizing a zirconia solid electrolyte as an oxygen ion-permeable solid electrode, and more particularly to an oxygen sensor suitable for detecting an oxygen-depleted state in a relatively low temperature environment, particularly in working environment such as construction sites, in tanks, etc.
As miniature, high-sensitivity oxygen sensors, limiting current-type oxygen sensors utilizing a zirconia solid electrolyte as an oxygen ion-permeable means have conventionally been known.
The zirconia solid electrolyte is, as is well known, a ceramic of ZrO.sub.2 (zirconia) containing CaO (calcia) or Y.sub.2 O.sub.3 (yttria), etc. as a stabilizer in the form of a solid solution. This zirconia solid electrolyte shows a high oxygen ion permeability at a constant voltage when heated to 350.degree. C. or higher. In other words, in an atmosphere containing oxygen, the zirconia solid electrolyte can selectively permit oxygen to pass therethrough. This solid electrolyte-type sensor can detect oxygen concentration in a gas being measured, by utilizing peculiar characteristics of such solid electrolyte.
In addition, the zirconia solid electrolyte has excellent heat resistance, corrosion resistance, thermal shock resistance, etc. Utilizing such advantages, the oxygen sensor made of a zirconia solid electrolyte is widely used for controlling combustion in boilers and furnaces, controlling air-fuel ratios and setting optimum conditions for cleaning exhaust gas in internal engines of automobiles, etc. and further detecting an oxygen-depleted state in such working environments as construction sites, in tanks, etc.
Solid electrolyte-type sensors utilized in such wide applications are generally classified into an oxygen concentration cell-type and an electrochemical pumping-type.
The oxygen concentration cell-type sensor generally has a structure in which a substrate made of a zirconia solid electrolyte is provided with porous Pt electrodes on both surfaces thereof. One electrode is in contact with a gas being measured while the other electrode is in contact with a reference gas having a known oxygen concentration, for instance, the air. By this structure, an oxygen concentration cell is formed, and the measurement of electromotive force of this oxygen concentration cell can lead to the detection of the oxygen concentration of the gas being measured.
On the other hand, the electrochemical pumping-type sensor is constructed such that the oxygen concentration of the gas being measured can be detected by utilizing an electrochemical pumping function. This electrochemical pumping function means that when voltage is applied to an oxygen ion-permeable zirconia solid electrolyte, the oxygen in the gas being measured is reduced to oxygen ions by a negative electrode, and these oxygen ions move through the solid electrolyte to a positive electrode, where they are oxidized to oxygen again, and it is discharged outside.
As a typical sensor utilizing the above electrochemical pumping function, a limiting current-type oxygen sensor is disclosed in Japanese Patent Publication No. 59-26895. This limiting current-type oxygen sensor will be explained as a first type sensor referring to FIG. 5.
In FIG. 5, the sensor comprises a planar oxygen ion-permeable solid electrolyte 21 provided with a porous internal electrode (negative electrode) 22 and a porous external electrode (positive electrode) 23 on both surfaces thereof. Fixed to the solid electrolyte 21 on the side of the internal electrode 22 is a gas diffusion adapter 24 provided with a gas diffusion pore 25 having a desired pore diameter for permitting an oxygen gas to go into an internal chamber 26 defined by the solid electrolyte 21 and the gas diffusion adapter 24. 27 denotes a DC power supply, and 28 denotes a current measurement circuit, one terminal of which is connected to a positive terminal of the DC power supply 27. 29 denotes a lead wire for connecting the internal electrode 22 to the negative terminal of the DC power supply 27, and 30 denotes a lead wire for connecting the external electrode 23 to the current measurement circuit 28.
In the limiting current-type oxygen sensor having the above structure, when a certain voltage is applied to the solid electrolyte 21 by the DC power supply 27 with the internal electrode 22 and the external electrode 23 biased negatively and positively, respectively, oxygen in the internal chamber 26 is electrochemically pumped to the outside through the solid electrolyte 21. In this process, the amount of oxygen diffused through the gas diffusion pore 25 from outside into the internal chamber 26 is controlled by the rate of oxygen diffusion through the gas diffusion pore 25, so that the amount of oxygen ions moving in the solid electrolyte 21 is kept constant. As a result, a constant limiting current proportional to the oxygen concentration in the gas being measured flows in the electric current measurement circuit 28. Thus, the oxygen concentration in the gas being measured can be known from the limiting current obtained by applying a constant voltage.
The limiting current obtained in such a process varies depending upon the size of the gas diffusion pore 25, namely an opening ratio (an area of opening/length of pore) of the gas diffusion pore 25. When the opening ratio is decreased by decreasing the pore diameter of the gas diffusion pore 25, the rate of the process is more controlled by oxygen diffusion through the gas diffusion pore 25, leading to the decrease in limiting current.
On the other hand, when the opening ratio is increased by increasing the pore diameter of the gas diffusion pore 25, the rate of process is less controlled by oxygen diffusion through the gas diffusion pore 25, leading to the increase in a limiting current. Apart from the influence by the area of the electrode, it is generally satisfied that the smaller the limiting current, the smaller the voltage applied to obtain such limiting current.
Japanese Patent Laid-Open No. 59-88653 discloses as a second type a limiting current-type oxygen sensor as shown in FIG. 6.
In FIG. 6, the oxygen sensor comprises a tubular oxygen ion-permeable solid electrolyte container (ZrO.sub.2 +Y.sub.2 O.sub.3) 32 provided with a gas diffusion pore 33 in a center portion of the bottom of the container 32. This solid electrolyte container 32 is provided, on both upper inner and outer surfaces, with a negative electrode 34 and a positive electrode 35. A top opening portion of the container 32 is lined with a metallized layer 36. 37 denotes an elongated lid member made of the same material as that of the container 32, an upper side wall of which is coated with a metallized layer 38. The solid electrolyte container 32 and the lid member 37 are in contact with each other between the metallized layers 36 and 38 via an annular seal member 39.
In this oxygen sensor of the second type, oxygen in a gas being measured is diffused from outside (a system to be measured) to a cavity defined by the solid electrolyte container 32 and the lid member 37 via the gas diffusion pore 33, and oxygen diffusion through the gas diffusion pore 33 determines a total oxygen diffusion rate of this sensor. Accordingly, a constant limiting current in proportion to the oxygen concentration of the gas being measured flows between the negative electrode 34 and the positive electrode 35, thereby enabling the detection of oxygen concentration.
Further, a limiting current-type oxygen sensor is known by Japanese Patent Laid-Open No. 57-48648.
In addition to the above limiting current-type oxygen sensors, oxygen sensors having electrochemical pumping mechanisms and oxygen concentration detection elements are also known.
Japanese Patent Laid-Open No. 57-97439 discloses as a third type an oxygen sensor shown in FIG. 7.
In FIG. 7, the oxygen sensor comprises an oxygen pumping element 45 constituted by a solid electrolyte 41 having a fine pore 44 substantially in a center thereof and electrodes 42, 43 formed on both surfaces of the solid electrolyte 41, and an oxygen detection element (oxygen concentration cell) 49 constituted by a solid electrolyte 46 and electrodes 47, 48 formed on both surfaces of the solid electrolyte 46. The oxygen pumping element 45 and the oxygen detection element 49 are fixed to each other via an annular conductive member 50 such that their planar surfaces are opposite to each other. An internal chamber 51 is defined by the electrodes 43, 48 and the annular conductive member 50. 52 denotes a DC power supply, 53 denotes an electromotive force meter and 54-57 denote lead wires. Incidentally, contact portions of the oxygen pumping element 45 and the oxygen detection element 49 may be sealed with a glass material.
When a constant voltage is applied to the oxygen pumping element 45 by the DC power supply 52, the internal chamber 51 is filled with a reference gas (air) by an oxygen pumping function. In this state, when the electrode 47 of the oxygen detection element 49 is brought into contact with a gas to be measured, an oxygen concentration cell is formed, whereby an electromotive force in proportion to a ratio of the oxygen concentration of the reference gas to the oxygen concentration of the gas being measured can be obtained from the element 49. The oxygen concentration of the gas being measured can be known by measuring this electromotive force by an electromotive force meter 53.
Japanese Patent Laid-Open No. 58-210560 discloses as a fourth type an oxygen sensor which has a similar structure to that shown in FIG. 6. Namely, the oxygen sensor of the fourth type comprises a tubular oxygen ion-permeable ceramic element provided with a gas diffusion pore in its bottom and two pairs of electrodes opposite to each other via the gas diffusion pore on both inner and outer surfaces of the bottom, and a ceramic lid member for sealing an open end of the tubular oxygen ion-permeable ceramic element, thereby providing an internal chamber therebetween. One pair of electrodes and a ceramic element portion therebetween function as a pumping cell, and another pair of electrodes and a ceramic element portion therebetween function as a sensor cell (oxygen concentration cell). The principle of oxygen concentration detection in the oxygen sensor of the fourth type is essentially the same as in that of the third type.
Japanese Patent Laid-Open No. 60-24445 discloses as a fifth type an automobile oxygen sensor as shown in FIG. 8.
In FIG. 8, the oxygen sensor comprises a cylindrical ceramic member 61 and a lean sensor (limiting current-type oxygen sensor capable of detecting oxygen only in a lean range of an air-fuel ratio) 65 attached to one open end of the cylindrical member 61. The lean sensor 65 is constituted by an oxygen ion-permeable solid electrolyte 62 having a gas diffusion pore 64 in its center, and electrodes 63a, 63b formed on both surfaces of the solid electrolyte 62. The oxygen sensor also comprises an oxygen pump 68 fixed in the cylindrical member 61 such that an internal chamber is defined between the lean sensor 65 and the oxygen pump 68. The oxygen pump 68 is constituted by an oxygen ion-permeable solid electrolyte 66 and electrodes 67a, 67b formed on both surfaces of the solid electrolyte 66. 69a and 69b denote lead wires, 70 and 72 denote stabilized DC power supplies. 71 denotes a current measurement circuit, and 73 denotes a heater contained in the cylindrical member 61.
In this oxygen sensor of the fifth type having the above structure, the internal chamber is filled with oxygen having a known concentration pumped by the oxygen pump 68, and a gas introduced through the gas diffusion pore 64. Accordingly, even when the gas being measured is in a rich state of an air-fuel ratio (oxygen-depleted state), the internal chamber is in a lean state (oxygen excess state). Therefore, a limiting current obtained by the lean sensor 65 can lead to the detection of oxygen concentration in the gas being measured.
However, the above-described solid electrolyte-type oxygen sensors have various problems as mentioned below.
In the case of the oxygen concentration cell-type sensor, since the reference gas is required as its essential element, the sensor volume is inevitably large, consuming much electric energy.
In the cases of the electrochemical pumping-type sensors exemplified by the first and second types, the reference gas is not needed, making it possible to miniaturize the sensor itself as compared to the above-described oxygen concentration cell-type sensor. However, it still suffers from the following peculiar problems. That is, to achieve the rate determination by oxygen diffusion, the adapter 24 having a gas diffusion pore 25 is needed as an additional element in the first type, and the lid member 37 and the seal member 39 are needed in the second type. Accordingly, these electrochemical pumping-type sensors have relatively complicated structures because of increased numbers of parts.
In the cases of the oxygen sensors having the electrochemical pump and the oxygen concentration cell exemplified by the third and fourth types, and the oxygen sensor of the fifth type having the electrochemical pump and the lean sensor, they are disadvantageous in complicated structure. In addition, constituent elements such as the electrochemical pump, the oxygen concentration cell and the lean sensor should be quality-controlled, but the control of their quality is rather difficult. As a result, unevenness in quality inevitably occurs among the sensors having such elements.
Also, in the above-described electrochemical pumping-type sensors, to increase response velocities by decreasing the heat capacities of the overall sensors, the sensors themselves should be miniaturized. In this case, the gas diffusion pores inevitably have reduced diameters in the above structures. However, in the electrochemical pumping-type sensors, the gas diffusion pores are directly exposed to a gas being measured. Accordingly, the fine diffusion pores are likely to be clogged with dust in the gas being measured, leading to the deterioration of their function as sensors.